How to minify css and make combined js file async

Hi I need to optimise the site . So I want

minify css

make combined js file async

I selected all the external css in Themes-> Layout -> my Layout and enabled Combine scripts. Then enabled Minify the markup and Compress scripts in System->Settings . But the css file is not minified.What will be the reason ?

Minify the markup is only for HTML code as explained in the help text below.
You should not activate Compress scripts as this is only for bad servers which are not able to send data gziped and requires a change in the htaccess also.

Minify the markup is only for HTML code as explained in the help text below.
You should not activate Compress scripts as this is only for bad servers which are not able to send data gziped and requires a change in the htaccess also.

then,what is the correct method to minify css and avoid render blocking in contao ?